Wednesday, October 26, 2011

This post allows you to navigate the American Community Survey Guide. If you don't know much about the ACS, we suggest you begin with the Introduction or at the top of the Frequently Asked Questions. If you're a practitioner, you may want to start with Using Multi-Year Estimates or Data Products...or the middle of the FAQ's. Don't miss the Useful Graphics!

Tuesday, March 22, 2011

4. If the same data appear in the 2010 Decennial Census and in the ACS, which should I use?

A. It depends. The decennial census is the most accurate source of data for counts of population and housing because it is based on a full enumeration. And, if the data you want are available from the 2010 Decennial Census, they should be used. However, as time goes on, the 2010 Decennial Census data will quickly become out-of-date in many places, and the ACS may become the better choice. Click to see other Frequently Asked Questions

ACS data products were initially designed to be comparable to Census 2000 long-form sample products. However, based on extensive input from data users, the ACS data products have been redesigned and expanded, and additional tables have been added to reflect new content in the ACS that was not included in the Census 2000 long form. There are some differences between the data products provided for the one-year, three-year, and five-year ACS data. Table 1 provides a comparison of the ACS data products with the Census 2000 long-form sample products. These data are available through American FactFinder (AFF) on the Census Bureau’s Web site, accessible either directly or as downloadable files.

The ACS was fully implemented in 2005, and one-year ACS data products are available for each year thereafter. Three-year ACS data products started in 2008, with the release of the 2005–2007 period estimates and subsequent releases for 2006–2008 and 2007–2009. The first five-year ACS data products for 2005–2009 were released at the end of 2010. Annual releases of one-year, three-year, and five-year ACS data products are planned for 2011 and each year thereafter.

ACS data products can be divided into two broad categories: tabulated data products, and microdata files. The tabulated data products contain precalculated estimates of frequently requested totals, percentages, means, medians, and ratios. The microdata files, known as Public Use Microdata Sample (PUMS) files, contain a sample of the individual person and housing data and can be used to produce estimates not already published in the tabulated data products.

One important difference between ACS data products and Census 2000 data products is the inclusion of 90-percent margins of error (MOE) for all estimates in ACS tabulated data products. The MOE is a measure of the precision or reliability of an estimate. The larger the margin of error for an estimate, the less reliable or precise is that estimate. These margins of error can also be used to calculate 90-percent confidence intervals, which indicate that data users can be 90-percent confident that the true population value will usually fall within the range defined by the confidence interval. The margins of error in ACS tabulated data products provide an important tool to help data users understand the reliability of ACS estimates and to draw appropriate conclusions from the data. Measures of reliability are not provided in ACS PUMS files but can be calculated using either a standard formula or the replicate weights that are included in the files.

Now under way, the 2010 census has sparked bitter partisanship. Some conservative Republicans, for example, have criticized the census as an unconstitutional intrusion on privacy; others warn that census participation is important for maintaining GOP power, since the count is used to apportion congressional seats and allocate federal money to cities and states. Liberal Democrats have been more supportive of census procedures, which for the first time will count same-sex couples. To raise response rates, the Census Bureau sent every household the same brief 10-question form and dropped use of the “long form” — a lengthy questionnaire seeking data on housing, transportation, education and income. The long form has been replaced by a separate, ongoing monthly survey that will provide timelier data, but from a smaller sample of households. Researchers generally hail the change but say it will cause some problems, at least initially.

Monday, March 14, 2011

For decades, the long-form sample from the decennial census served as the only source of detailed social and demographic statistics for small areas and population groups in the United States. The long-form sample, however, had two major shortcomings.

First, since the information was collected as part of the census count, it was an added burden to the selected respondents and complicated the data collection effort by the Census Bureau. By 1990, the mail response rate for long-form recipients had dropped significantly below the rate for short-form recipients, increasing the costs of nonresponse follow-up (NRFU) and degrading the quality of the long-form data. In addition, because the census count is the Census Bureau’s top priority, the editing, processing, and tabulation of long-form data had to take a back seat to the full count enumeration, delaying the release of long-form data for two or three years after the actual enumeration.

Second, perhaps the biggest drawback of the census long-form sample is that the data were collected only once every ten years. The sample provided a snapshot of one point in time for the decade and quickly became out-of-date in many communities throughout the nation. Couple this with the delay in release of the long-form data, and it became increasingly apparent that an alternative to the census long form was needed.

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The Guide to the ACS is a section of the forthcoming second edition of the Encyclopedia of the U.S. Census from CQ Press, edited by Margo J. Anderson, Constance F. Citro, and Joseph J. Salvo. Read about the Encyclopedia of the U.S. Census Second Edition: From the Constitution to the American Community Survey.